1. Field of the Invention
The present invention relates to dies for applying insulation enamel coatings to wires (hereinafter referred to as “coating dies”), and particularly to dies for coating flat wires (which are advantageously used to increase the filling factor (space factor) of coils). Furthermore, the invention relates to methods for manufacturing enameled flat wires using the invented dies.
2. Description of Related Art
Enameled wires (enamel covered insulated wires) are widely used for coil wires in electrical equipment such as motors and transformers. Such enameled wires are formed by covering an insulation coating around a metal conductor having a desired cross section (such as circular or rectangular) depending on the shape and application of the coil. With the current trend toward small and high power vehicle motors (such as alternators), increasing demands exist for enameled flat wires (having a rectangular cross section) that can be wound into a coil more densely than enameled round wires (having a circular cross section). In order to achieve more accurate coil winding and a higher coil filling factor, there also exist demands for thinner and more uniform insulation enamel coatings.
Known coating dies for applying an insulation enamel (varnish) around a conductor are classified into die assemblies and solid dies. Die assemblies will be explained first. FIG. 1 is a schematic illustration showing, an example of a die part of a die assembly for flat wire coating, front, side and top views and an enlarged view of the principal portion. FIG. 2 is a schematic illustration showing a top view of a die assembly assembled from four die parts shown in FIG. 1.
As illustrated in FIG. 1, the die part 1 includes a cylindrical die body 2 and an approximately cubic die base 3. The die base 3 has an insertion hole 4 for insertion of the die body 2 of another die part 1. The cylindrical surface of the die body 2 has numerous annular coating grooves 5 for supplying an insulation varnish therethrough in order to apply the varnish around a flat wire conductor. As illustrated in FIG. 2, a die assembly 6 for flat wire coating is assembled from four FIG. 1 die parts 1. The space surrounded by the four die parts 1 forms a die hole 7 for insertion of a flat wire conductor in order to apply an insulation varnish around the conductor.
Such die assemblies for enameling flat wires (also referred to as “flat wire coating die assemblies”) as shown in FIGS. 1 and 2 have an advantage of being adaptable to different size flat wire conductors. That is, the die hole size of a die assembly can be changed by adjusting the insertion depth of each die body into the corresponding die base hole when assembling the die assembly from a set of die parts (four parts in FIG. 2). Also, die assemblies have another advantage that a die assembly can be readily assembled and disassembled, and therefore it can be replaced with another die assembly without the need for cutting a flat wire conductor.
Although die assemblies have the above advantage of being adaptable to different size flat wire conductors, they have the following disadvantage. It is difficult to accurately and controllably adjust the gap between the die hole of a die assembly and a flat wire conductor to be coated. Therefore, die assemblies cannot be used to form enameled wires requiring a very small allowable error (or tolerance) for the coating thickness or the finished dimensions.
Next, solid dies will be explained. Solid dies include: A die body (typically, approximately cylindrical) and a die hole of a fixed shape and dimensions formed through the die body. As just noted, the die hole of solid dies has a fixed shape and dimensions. Therefore, the gap between a die hole and a conductor to be coated can be accurately adjusted, and as a result an insulation varnish can be uniformly applied around the conductor. Thus, solid dies are suitably used to manufacture enameled wires requiring a small tolerance for the coating thickness or the finished dimensions.
Besides the above two types of coating dies, JP-A 2003-297164 discloses an assemblable/disassemblable coating die which combines the advantage of the accurate coating capability of solid dies with the advantage of the assemblability/disassemblability of die assemblies. Needless to say, the die hole of solid dies for enameled flat wires (also referred to as “flat wire coating solid dies”) has a rectangular cross section.
There still remains a yet-unsolved problem shared by all of the above listed types of coating dies—circumferential nonuniformity in coating thickness. More specifically, a coating formed around a flat wire by a conventional coating die is prone to be selectively thinner on each rounded corner of the flat wire and thicker on the both sides of the corner. That is, an undesirable local thickening/thinning phenomenon (what is called a dog-bone phenomenon, see later-described FIG. 7) is prone to occur on each rounded corner of the flat wire. Conventional thinking has been that this phenomenon is caused by the surface tension difference of an applied insulation varnish and is unavoidable in coatings formed by conventional coating dies.
There has been a strong demand for solutions to this problem because thickness nonuniformities of an insulation coating (such as a dog-bone surface) may degrade the high voltage electrical insulation and also may make accurate coil winding difficult. In order to prevent such a undesirable local thickening/thinning phenomenon, JP-A 2004-134113 discloses an insulated flat wire in which each corner of a flat wire conductor to be coated is shaped in such a manner that the cross section has a polygonal shape (i.e., has two or more vertices). The total number of vertices per flat wire conductor is 8 or more, and each vertex angle is 120° or more. According to this JP-A 2004-134113, the corners of the flat wire conductor are not exposed to the coating surface, the coating thicknesses on the corners of the conductor are sufficiently thick, and the resulting insulated flat wire has excellent electrical insulation properties.
As described above, there is an increasing demand for enameled flat wires in terms of the filling factor of coil windings as well as for uniform insulation enamel coatings in terms of breakdown voltage and coil winding accuracy. The technology disclosed in the above JP-A 2004-134113 has an advantage that an insulation coating can be uniformly formed. However, the cross section of the flat wire conductor used in this technology is close to a race-track shape (or an elongated circle) rather than a rectangle; therefore, coils wound from such a flat wire may not have a satisfactorily high filling factor.
As for solid dies, they can be advantageously used to form uniform insulated round wires. However, even solid dies cannot be effectively used to form uniform enameled flat wires because the above-described undesirable local thickening/thinning (dog-bone) phenomena inevitably tends to occur. And, there have not yet been proposed any practical solutions to this problem of local thickening/thinning (dog-bone) phenomenon accompanying the flat wire enameling processes.